Preparation and Characterizations of Three Dimensional Nanostructured Carbon Composites as Anode Materials in Li-ion Batteries

Abstract

The demand for higher specific energy Li-ion batteries for applications such as electric vehicles, next generation electronic devices, and renewable energy sources motivates the research toward electrode materials with larger specific capacities. Among several materials proposed to replace currently available commercial graphite (372 mA h g-1) at the anode electrode, silicon and cobalt oxides have been widely studied as anode materials for use in high performance Li-ion batteries due to their high theoretical capacity of 4200 mA h g-1 and 890 mA h g-1, respectively. However, silicon and cobalt oxides have a few problems, including a large volume change during Li insertion/extraction and a poor electrical conductivity. In order to overcome these problems, several studies focused on the possibility of hybridizing silicon and cobalt oxide nanoparticles (NPs) with electrically conducting carbon metrics. Especially, three dimensional (3-D) nanostructure of the ordered mesoporous carbon (OMC) can shorten the Li ion diffusion path and buffer the stress induced by volume expansion, which leads to high specific capacities. In this dissertation, the 3-D nanostructured metal or metal oxides/OMC composite prepared by introducing a solvent evaporation induced co-self-assembly strategy are proposed as anode materials and electrode for the Li-ion batteries. This facile, scalable, and one-pot synthesis is using a triblock copolymer as soft template, a resorcinol-formaldehyde polymer as carbon precursor and a metal precursor. And also the preparation of 3-D nanostructured electrode is possible through the combination with a hard template. Details are as follows: Firstly, the Si NPs trapped in OMC (Si/OMC) composite is prepared by a one-step self-assembly with solvent evaporation. The commercial Si NPs captured by the F127/phenolic resol via hydrogen bonding were well-dispersed in the OMC framework through carbonization. The improved electrochemical performances of the composite can be ascribed to the buffering effect of spaces formed in ordered pore channels during the volume expansion of silicon and the rapid movement of lithium ions through the uniform cylindrical pore structure of the mesopores. Secondly, the Co3O4 NPs embedded in the OMC (Co3O4/OMC) composite, in which Co3O4 NPs with an average size of about below 10 nm are homogeneously embedded in the OMC framework are fabricated. The high capacities of the Co3O4/OMC composite can be related to the increased charge storage within the polymer film on the surface of small and well-dispersed Co3O4 NPs prepared by a thermal decomposition of a Co2+ confined in a polymer framework. Finally, the 3-D mesoporous carbon nanotube filled with Co3O4 NPs (Co3O4/MCNT) electrode are prepared by the dual templating method using amphiphilic surfactants (F127) as soft-template and anodic aluminum oxide (AAO) membranes as hard template. The Co3O4/MCNT composite exhibit a high reversible capacity because 3-D structured carbon framework allow direct electronic pathways for efficient charge transport and well-dispersed Co3O4 NPs in the MCNT is useful to diminish of surface resistance by formation stable SEI layer.